The present invention relates to a laminated material for the manufacture of packing containers with good gas and aroma barrier properties, such a material comprising a base layer and a barrier layer serving as a gas and aroma barrier. The invention also relates to a method for the manufacture of the laminated material.
In packing technology consumer packages of non-returnable character have been used for a long time for the packaging and transporting of liquid foodstuffs. The material in these so-called non-returnable or one-way packages usually consists of several material layers laminated to one another whose number and composition may vary so as to provide the package in each individual case with the required mechanical and other protective properties required for the particular foodstuff. A conventional non-returnable package for milk usually is manufactured from a packing material comprising a base layer consisting of paper or cardboard which furnishes the package with the necessary mechanical strength characteristics, and one or more outer coatings of thermoplastics, e.g. polythene, which on the one hand renders the package liquid-tight and on the other hand makes it possible for the package to be made permanent in its intended final geometric shape by means of so-called heat-sealing which consists in surface-fusing of the plastic coatings facing one another by means of heat and pressure so as to form liquid-tight, mechanically strong sealing joints during the manufacture of the packages. However, this packing material lacks tightness properties in respect of gases as the same time as it is pervious for, or has insufficient tightness properties towards, certain types of aroma substances, and is unsuitable, therefore, as packing material in packages intended for oxygen gas sensitive foodstuffs, e.g. juice which requires packages with good gas and aroma barrier properties. To furnish the packages with the necessary tightness properties towards gases and aroma substances the material in these packages is provided, therefore, with at least one further layer of a material with good gas and aroma barrier properties, usually a metal foil (Al foil) applied to the inside of the base layer which in addition to providing the packages with the necessary tightness properties also makes the material inductively heatable and thus allows an effective heat sealing by means of induction heating. It is a serious disadvantage of this known packing material, though, that an Al foil, because of its low extensibility, easily breaks or cracks along particularly exposed regions or so-called crosses during the conversion of the material to packages where the strains may become so great that thermoplastic coatings included in the material also rupture and so bring about seriously impaired tightness properties of the finished package.
It is also known to provide packing material with a barrier layer serving as a gas and aroma barrier of a polymer material, e.g. ethylene-vinyl alcohol copolymer (EVOH), but these barrier layers as a rule are moisture-sensitive and rapidly lose their tightness properties if they are exposed to moisture.
It is an object of the present invention, therefore, to provide directions regarding a new material for the manufacture of packing containers with good gas and aroma barrier properties without, or with appreciably reduced, risks of crack formation and lost barrier properties during the conversion of the material to packing containers associated therewith.
It is a further object to provide a packing material free from moisture-sensitive gas and aroma barrier layers.
It is a further object to provide directions regarding a packing material which in addition to good gas and aroma barrier properties is also pervious to microwave radiation and thus can be used for the manufacture of microwave-heatable packing containers.
These objects are achieved in accordance with the present invention in that a laminated material of the type described in the introduction has been given the characteristic that the barrier layer serving as gas and aroma barrier consists of inorganic material and is of a thickness of only 50-500 xc3x85ngstrxc3x6m (xc3x85).
Quite surprisingly it has been found, in fact, that the inorganic material layer, which preferably consists of a silicon compound, e.g. silicon dioxide (SiO2) or silicon nitride (Si3N4), in spite of its small material thickness possesses extraordinarily good gas and aroma barrier properties at the same time as, thanks to the small material thickness, it is sufficiently flexible and extensible to make possible a conversion of the material to packing containers without any danger of cracking or breaking even in the most exposed material regions. A further advantage which is obtained by the preferred barrier layer mentioned here is that it is practically completely inert and thus neither affects nor is affected by the particular contents in the finished package.
Further objects of the present invention, moreover, have been given the characteristics which are evident from the subsidiary claims.